GPS radio occultation with GRACE : Atmospheric profiling utilizing the zero difference technique

Radio occultation events recorded on 28–29 July 2004 by the " BlackJack " GPS receiver aboard the GRACE-B satellite are analyzed. The high stability of the receiver clock allows for the implementation of a zero difference technique rendering clock calibrations with concurrent observations of a reference GPS satellite unnecessary. Thereby the usage of ionospheric correction procedures for the derivation of excess phase paths is eliminated. Comparisons of refractivity profiles obtained by zero differencing with results calculated with the standard single difference method yield good agreement with an average fractional refractivity deviation below 1% and an one-sigma standard deviation of 2–3% at altitudes below 35 km. Both data sets are consistent with co-located ECMWF meteorological analyses.